Direct epoxidation process using a mixed catalyst system

ABSTRACT

Catalyst mixtures useful for the direct epoxidation of olefins are disclosed. The catalyst mixture comprises a titanium or vanadium zeolite and a supported catalyst comprising a noble metal, bismuth, and a carrier. The invention includes a process for producing an epoxide comprising reacting an olefin, hydrogen and oxygen in the presence of the catalyst mixture. The process results in significantly reduced alkane byproduct formed by the hydrogenation of olefin.

FIELD OF THE INVENTION

This invention relates to a catalyst system and its use in theproduction of epoxides from hydrogen, oxygen, and olefins.

BACKGROUND OF THE INVENTION

Many different methods for the preparation of epoxides have beendeveloped. Generally, epoxides are formed by the reaction of an olefinwith an oxidizing agent in the presence of a catalyst. Ethylene oxide iscommercially produced by the reaction of ethylene with oxygen over asilver catalyst. Propylene oxide is commercially produced by reactingpropylene with an organic hydroperoxide oxidizing agent, such asethylbenzene hydroperoxide or tert-butyl hydroperoxide. This process isperformed in the presence of a solubilized molybdenum catalyst, see U.S.Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, seeU.S. Pat. No. 4,367,342.

Besides oxygen and alkyl hydroperoxides, hydrogen peroxide is also auseful oxidizing agent for epoxide formation. U.S. Pat. Nos. 4,833,260,4,859,785, and 4,937,216, for example, disclose olefin epoxidation withhydrogen peroxide in the presence of a titanium silicate catalyst.

Much current research is conducted in the direct epoxidation of olefinswith oxygen and hydrogen. In this process, it is believed that oxygenand hydrogen react in situ to form an oxidizing agent. Many differentcatalysts have been proposed for use in the direct epoxidation process.Typically, the catalyst comprises a noble metal and a titanosilicate.For example, JP 4-352771 discloses the formation of propylene oxide frompropylene, oxygen, and hydrogen using a catalyst containing a Group VIIImetal such as palladium on a crystalline titanosilicate. The Group VIIImetal is believed to promote the reaction of oxygen and hydrogen to forma hydrogen peroxide in situ oxidizing agent. U.S. Pat. No. 6,498,259describes a catalyst mixture of a titanium zeolite and a supportedpalladium complex, where palladium is supported on carbon, silica,silica-alumina, titania, zirconia, and niobia. Other direct epoxidationcatalyst examples include gold supported on titanosilicates, see forexample PCT Intl. Appl. WO 98/00413.

One disadvantage of the described direct epoxidation catalysts is thatthey are prone to produce non-selective byproducts such as glycols orglycol ethers formed by the ring-opening of the epoxide product oralkane byproduct formed by the hydrogenation of olefin. U.S. Pat. No.6,008,388 teaches that the selectivity for the direct olefin epoxidationprocess is enhanced by the addition of a nitrogen compound such asammonium hydroxide to the reaction mixture. U.S. Pat. No. 6,399,794teaches the use of ammonium bicarbonate modifiers to decrease theproduction of ring-opened byproducts. U.S. Pat. No. 6,005,123 teachesthe use of phosphorus, sulfur, selenium or arsenic modifiers such astriphenylphosphine or benzothiophene to decrease the production ofpropane. U.S. Pat. No. 7,026,492 discloses that the presence of carbonmonoxide, methylacetylene, and/or propadiene modifier givessignificantly reduced alkane byproduct. In addition, co-pending U.S.patent application Ser. No. 11/489,086 discloses that the use of alead-modified palladium-containing titanium or vanadium zeolite reducesalkane byproduct formation.

As with any chemical process, it is desirable to attain still furtherimprovements in the epoxidation methods and catalysts. We havediscovered a new catalyst and its use in olefin epoxidation.

SUMMARY OF THE INVENTION

The invention is a catalyst mixture that comprises a titanium orvanadium zeolite and a supported catalyst comprising a noble metal,bismuth and a carrier. The catalyst mixture is useful in olefinepoxidation reactions. Thus, the invention includes an olefinepoxidation process that comprises reacting an olefin, hydrogen andoxygen in the presence of the catalyst mixture. This processsurprisingly gives significantly reduced alkane byproduct formed by thehydrogenation of olefin.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst mixture of the invention comprises (1) a titanium orvanadium zeolite and (2) a supported catalyst. Titanium or vanadiumzeolites comprise the class of zeolitic substances wherein titanium orvanadium atoms are substituted for a portion of the silicon atoms in thelattice framework of a molecular sieve. Such substances, and theirproduction, are well known in the art. See for example, U.S. Pat. Nos.4,410,501 and 4,666,692.

Suitable titanium or vanadium zeolites are those crystalline materialshaving a porous molecular sieve structure with titanium or vanadiumatoms substituted in the framework. The choice of titanium or vanadiumzeolite employed will depend upon a number of factors, including thesize and shape of the olefin to be epoxidized. For example, it ispreferred to use a relatively small pore titanium or vanadium zeolitesuch as a titanium silicalite if the olefin is a lower aliphatic olefinsuch as ethylene, propylene, or 1-butene. Where the olefin is propylene,the use of a TS-1 titanium silicalite is especially advantageous. For abulky olefin such as cyclohexene, a larger pore titanium or vanadiumzeolite such as a zeolite having a structure isomorphous with zeolitebeta may be preferred.

Particularly preferred titanium or vanadium zeolites include the classof molecular sieves commonly referred to as titanium silicalites,particularly “TS-1” (having an MFI topology analogous to that of theZSM-5 aluminosilicate zeolites), “TS-2” (having an MEL topologyanalogous to that of the ZSM-11 aluminosilicate zeolites), “TS-3” (asdescribed in Belgian Pat. No. 1,001,038), and Ti-MWW (having an MELtopology analogous to that of the MWW aluminosilicate zeolites).Titanium-containing molecular sieves having framework structuresisomorphous to zeolite beta, mordenite, ZSM-48, ZSM-12, SBA-15, TUD,HMS, and MCM-41 are also suitable for use. The titanium zeolitespreferably contain no elements other than titanium, silicon, and oxygenin the lattice framework, although minor amounts of boron, iron,aluminum, sodium, potassium, copper and the like may be present.

Preferred titanium zeolites will generally have a compositioncorresponding to the following empirical formula xTiO₂ (1−x)SiO₂ where xis between 0.0001 and 0.5000. More preferably, the value of x is from0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of thezeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1to 60:1). The use of relatively titanium-rich zeolites may also bedesirable.

The catalyst mixture of the invention also comprises a supportedcatalyst that comprises a noble metal, bismuth and a carrier. Thecarrier is preferably a porous material. Carriers are well-known in theart. For instance, the carrier can be inorganic oxides, clays, carbon,and organic polymer resins. Preferred inorganic oxides include oxides ofGroup 2, 3, 4, 5, 6, 13, or 14 elements. Particularly preferredinorganic oxide carriers include silica, alumina, silica-aluminas,titania, zirconia, niobium oxides, tantalum oxides, molybdenum oxides,tungsten oxides, amorphous titania-silica, amorphous zirconia-silica,amorphous niobia-silica, and the like. The carrier may be a zeolite, butis not a titanium or vanadium zeolite. Preferred organic polymer resinsinclude polystyrene, styrene-divinylbenzene copolymers, crosslinkedpolyethyleneimines, and polybenzimidizole. Suitable carriers alsoinclude organic polymer resins grafted onto inorganic oxide carriers,such as polyethylenimine-silica. Preferred carriers also include carbon.Particularly preferred carriers include carbon, silica, silica-aluminas,zirconia, niobia, and titania (in particular anatase titanium dioxide).

Preferably, the carrier has a surface area in the range of about 1 toabout 700 m²/g, most preferably from about 10 to about 500 m²/g.Preferably, the pore volume of the carrier is in the range of about 0.1to about 4.0 mL/g, more preferably from about 0.5 to about 3.5 mL/g, andmost preferably from about 0.8 to about 3.0 mL/g. Preferably, theaverage particle size of the carrier is in the range of about 0.1 μm toabout 0.5 inch, more preferably from about 1 μm to about 0.25 inch, andmost preferably from about 10 μm to about 1/16 inch. The preferredparticle size is dependent upon the type of reactor that is used, forexample, larger particle sizes are preferred for a fixed bed reaction.The average pore diameter is typically in the range of about 10 to about1000 Å, preferably about 20 to about 500 Å, and most preferably about 50to about 350 Å.

The supported catalyst also contains a noble metal and bismuth. Whileany of the noble metals can be utilized (i.e., gold, silver, platinum,palladium, iridium, ruthenium, osmium), either alone or in combination,palladium, platinum, gold, a palladium/platinum, or a palladium/goldcombination are particularly desirable. Palladium is most preferred.

Typically, the amount of noble metal present in the supported catalystwill be in the range of from 0.01 to 10 weight percent, preferably 0.01to 4 weight percent. There are no particular restrictions regarding thechoice of noble metal compound or complex used as the source of noblemetal in the supported catalyst. For example, suitable compounds includethe nitrates, sulfates, halides (e.g., chlorides, bromides),carboxylates (e.g. acetate), oxides, and amine complexes of the noblemetal.

Similarly, the oxidation state of the noble metal is not consideredcritical. The noble metal may be in an oxidation state anywhere from 0to +4 or any combination of such oxidation states. To achieve thedesired oxidation state or combination of oxidation states, the noblemetal compound after being introduced into the supported catalyst may befully or partially pre-reduced. Satisfactory catalytic performance can,however, be attained without any pre-reduction.

The supported catalyst of the invention also contains bismuth. Thetypical amount of bismuth present in the supported catalyst will be inthe range of from about 0.001 to 5 weight percent, preferably 0.01 to 2weight percent. Preferably, the weight ratio of noble metal to bismuthin the catalyst is in the range of 0.1 to 10. While the choice ofbismuth compound used as the bismuth source in the supported catalyst isnot critical, suitable compounds include bismuth carboxylates (e.g.,acetate, citrate), halides (e.g., chlorides, bromides, iodides),oxyhalides (e.g., oxychloride), carbonates, nitrates, phosphates,oxides, and sulfides. The bismuth may be added to the carrier before,during, or after noble metal addition.

Any suitable method may be used for the incorporation of the noble metaland bismuth into the supported catalyst. For example, the noble metaland bismuth may be supported on the carrier by impregnation,ion-exchange, or incipient wetness techniques. For example, the noblemetal may be supported on the zeolite or the carrier by impregnation orby ion-exchange with, for example, palladium tetraammine chloride. Theorder of addition of noble metal and bismuth to the carrier is notconsidered critical. However, it is preferred to add the bismuthcompound at the same time that the noble metal is introduced.

After noble metal and bismuth incorporation, the supported catalyst isrecovered. Suitable supported catalyst recovery methods includefiltration and washing, rotary evaporation and the like. The supportedcatalyst is typically dried prior to use in epoxidation. The dryingtemperature is preferably from about 50° C. to about 200° C. Thesupported catalyst may additionally comprise a binder or the like andmay be molded, spray dried, shaped or extruded into any desired formprior to use in epoxidation.

After supported catalyst formation, the supported catalyst may beoptionally thermally treated in a gas such as nitrogen, helium, vacuum,hydrogen, oxygen, air, or the like. The thermal treatment temperature istypically from about 20° C. to about 800° C. It is preferred tothermally treat the supported catalyst in the presence of anoxygen-containing gas at a temperature from about 200° C. to 700° C.,and optionally reduce the supported catalyst in the presence of ahydrogen-containing gas at a temperature from about 20° C. to 600° C.

In the epoxidation process of the invention, the titanium or vanadiumzeolite and the supported catalyst may be used as a mixture of powdersor as a mixture of pellets. In addition, the titanium or vanadiumzeolite and supported catalyst may also be pelletized or extrudedtogether prior to use in epoxidation. If pelletized or extrudedtogether, the catalyst mixture may additionally comprise a binder or thelike and may be molded, spray dried, shaped or extruded into any desiredform prior to use in epoxidation. The weight ratio of titanium orvanadium zeolite: supported catalyst is not particularly critical.However, a titanium or vanadium zeolite: supported catalyst ratio of0.01-100 (grams of titanium or vanadium zeolite per gram of supportedcatalyst) is preferred, with a ratio of 1 to 20 more preferred, and aratio of 5 to 15 most preferred.

The epoxidation process of the invention comprises contacting an olefin,oxygen, and hydrogen in the presence of the catalyst mixture. Suitableolefins include any olefin having at least one carbon-carbon doublebond, and generally from 2 to 60 carbon atoms. Preferably the olefin isan acyclic alkene of from 2 to 30 carbon atoms; the process of theinvention is particularly suitable for epoxidizing C₂-C₆ olefins. Morethan one double bond may be present, as in a diene or triene forexample. The olefin may be a hydrocarbon (i.e., contain only carbon andhydrogen atoms) or may contain functional groups such as halide,carboxyl, hydroxyl, ether, carbonyl, cyano, or nitro groups, or thelike. The process of the invention is especially useful for convertingpropylene to propylene oxide.

Oxygen and hydrogen are also required for the epoxidation process.Although any sources of oxygen and hydrogen are suitable, molecularoxygen and molecular hydrogen are preferred.

Epoxidation according to the invention is carried out at a temperatureeffective to achieve the desired olefin epoxidation, preferably attemperatures in the range of 0-250° C., more preferably, 20-100° C. Themolar ratio of hydrogen to oxygen can usually be varied in the range ofH₂:O₂=1:10 to 5:1 and is especially favorable at 1:5 to 2:1. The molarratio of oxygen to olefin is usually 2:1 to 1:20, and preferably 1:1 to1:10. A carrier gas may also be used in the epoxidation process. As thecarrier gas, any desired inert gas can be used. The molar ratio ofolefin to carrier gas is then usually in the range of 100:1 to 1:10 andespecially 20:1 to 1:10.

As the inert gas carrier, noble gases such as helium, neon, and argonare suitable in addition to nitrogen and carbon dioxide. Saturatedhydrocarbons with 1-8, especially 1-6, and preferably with 1-4 carbonatoms, e.g., methane, ethane, propane, and n-butane, are also suitable.Nitrogen and saturated C₁-C₄ hydrocarbons are the preferred inertcarrier gases. Mixtures of the listed inert carrier gases can also beused.

Specifically in the epoxidation of propylene, propane can be supplied insuch a way that, in the presence of an appropriate excess of carriergas, the explosive limits of mixtures of propylene, propane, hydrogen,and oxygen are safely avoided and thus no explosive mixture can form inthe reactor or in the feed and discharge lines.

The amount of catalyst used may be determined on the basis of the molarratio of the titanium contained in the titanium zeolite to the olefinthat is supplied per unit time. Typically, sufficient catalyst ispresent to provide a titanium/olefin per hour molar feed ratio of from0.0001 to 0.1.

Depending on the olefin to be reacted, the epoxidation according to theinvention can be carried out in the liquid phase, the gas phase, or inthe supercritical phase. When a liquid reaction medium is used, thecatalyst mixture is preferably in the form of a suspension or fixed-bed.The process may be performed using a continuous flow, semi-batch orbatch mode of operation.

If epoxidation is carried out in the liquid (or supercritical orsubcritical) phase, it is advantageous to work at a pressure of 1-100bars and in the presence of one or more solvents. Suitable solventsinclude any chemical that is a liquid under reaction conditions,including, but not limited to, oxygenated hydrocarbons such as alcohols,ethers, esters, and ketones, aromatic and aliphatic hydrocarbons such astoluene and hexane, nitriles such as acetonitrile, liquid CO₂ (in thesupercritical or subcritical state), and water. Preferable solventsinclude water, liquid CO₂, and oxygenated hydrocarbons such as alcohols,ethers, esters, ketones, and the like. Preferred oxygenated solventsinclude lower aliphatic C₁-C₄ alcohols such as methanol, ethanol,isopropanol, and tert-butanol, or mixtures thereof, and water.Fluorinated alcohols can be used. It is particularly preferable to usemixtures of the cited alcohols with water.

If epoxidation is carried out in the liquid (or supercritical) phase, itis advantageous to use a buffer. The buffer will typically be added tothe solvent to form a buffer solution. The buffer solution is employedin the reaction to inhibit the formation of glycols or glycol ethersduring epoxidation. Buffers are well known in the art.

Buffers useful in this invention include any suitable salts of oxyacids,the nature and proportions of which in the mixture, are such that the pHof their solutions may range from 3 to 11, preferably from 4 to 9 andmore preferably from 5 to 8. Suitable salts of oxyacids contain an anionand cation. The anion portion of the salt may include anions such asphosphate, sulfate, carbonate, bicarbonate, carboxylates (e.g., acetate,phthalate, and the like), citrate, borate, hydroxide, silicate,aluminosilicate, or the like. The cation portion of the salt may includecations such as ammonium, alkylammoniums (e.g., tetraalkylammoniums,pyridiniums, and the like), alkali metals, alkaline earth metals, or thelike. Cation examples include NH₄, NBu₄, NMe₄, Li, Na, K, Cs, Mg, and Cacations. More preferred buffers include alkali metal phosphate andammonium phosphate buffers. Buffers may preferably contain a combinationof more than one suitable salt. Typically, the concentration of bufferin the solvent is from about 0.0001 M to about 1 M, preferably fromabout 0.001 M to about 0.3 M. The buffer useful in this invention mayalso include the addition of ammonia gas to the reaction system.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

EXAMPLE 1 Preparation of Pd—Bi/TiO₂, Pd—Bi—Au/TiO₂, and Pd—Au/TiO₂Catalysts

Catalyst 1A (Pd—Bi/TiO₂):

An aqueous solution of disodium palladium tetrachloride (1.11 g, 19.7wt. % Pd) is added to a solution of bismuth nitrate (0.35 g Bi(NO₃)₃dissolved in 15 mL, 2.56 M solution of nitric acid, 16.6% by volume of70% HNO₃). The Pd—Bi solution is then added by incipient wetness tospray dried titania (20 g, 30 micron size, 40 m²/g, calcined in air at700° C.). The solids are calcined in air in a muffle furnace by heatingat 10° C./min to 110° C. for 4 hours and then at 2° C./min to 300° C.for 4 hours. These calcined solids are then washed twice with an aqueoussodium bicarbonate solution (40 mL, containing 0.9 g NaHCO₃), followedby deionized water (40 mL, five times). The washed solids are vacuumdried (20 torr) at 50° C. for 16 hours and then calcined in a mufflefurnace by heating at 10° C./min to 110° C. for 4 hours and then heatingat 2° C./min to 600° C. for 4 hours. The solids are then transferred toa quartz tube and treated with a 4 vol. % hydrogen in nitrogen stream at100° C. for 1 hour (100 cc/hr), followed by nitrogen for 30 minuteswhile cooling from 100° C. to 30° C. to produce Catalyst 1A. Catalyst 1Acontains 0.94 wt. % Pd, 0.64 wt. % Bi, 58 wt. % Ti and less than 100 ppmNa.

Catalyst 1B (Pd—Bi—Au/TiO₂):

An aqueous solution of disodium palladium tetrachloride (1.33 g, 19.7wt. % Pd) and an aqueous solution of sodium tetrachloroaurate (0.83 g,19.8 wt. % Au) are added to a solution of bismuth nitrate (0.81 gBi(NO₃)₃ dissolved in 18 mL, 2.56 M solution of nitric acid, 16.6% byvolume of 70% HNO₃). The Pd—Bi—Au solution is then added by incipientwetness to spray dried titania (25 g, 30 micron size, 40 m²/g, calcinedin air at 700° C.). The solids are calcined in air in a muffle furnaceby heating at 10° C./min to 110° C. for 4 hours and then at 2° C./min to300° C. for 4 hours. These calcined solids are then washed twice with anaqueous sodium bicarbonate solution (50 mL, containing 1.13 g NaHCO₃),followed by deionized water (40 mL, five times). The washed solids arevacuum dried (20 torr) at 50° C. for 16 hours and then calcined in amuffle furnace by heating at 10° C./min to 110° C. for 4 hours and thenheating at 2° C./min to 600° C. for 4 hours. The solids are thentransferred to a quartz tube and treated with a 4 vol. % hydrogen innitrogen stream at 100° C. for 1 hour (100 cc/hr), followed by nitrogenfor 30 minutes while cooling from 100° C. to 30° C. to produce Catalyst1B. Catalyst 1B contains 0.8 wt. % Pd, 0.6 wt. % Au, 0.64 wt. % Bi, and58 wt. % Ti.

Comparative Catalyst 1C (Pd—Au/TiO₂):

Aqueous sodium tetrachloro aurate (16.54 g, 19.95 wt. % Au) and aqueousdisodium tetrachloro palladate (27.86 g, 19.74 wt. % Pd) are added to1.2 L of deionized water with swirling in a round-bottom flask. To thissolution, sodium bicarbonate (12.5 g) is added as a powder, followed byspray dried TiO₂ (500 g, 35 micron average size, 43 m²/g, air calcinedat 700° C.). The pH of the slurry is adjusted to 7.3 by adding solidportions of sodium bicarbonate (approximately 100 g is required) and thereaction slurry is agitated by rotation of the flask at 25 rpm at a 45degree angle for 18 hours at 23° C. The solids are then filtered, washedonce with deionized water (1.2 L), and calcined in air in a mufflefurnace by heating at 10° C./min to 110° C. for 4 hours and then at 2°C./min to 300° C. for 4 hours. These calcined solids are then washedwith deionized water (1.2 L) eight times. The washed solids are calcinedin a muffle furnace by heating at 10° C./min to 110° C. for 4 hours andthen heating at 2° C./min to 600° C. for 4 hours. The solids are thentransferred to a quartz tube and treated with a 4 vol. % hydrogen innitrogen stream at 100° C. for 1 hour (100 cc/hr), followed by nitrogenfor 30 minutes while cooling from 100° C. to 30° C. to produceComparative Catalyst 1B. Comparative Catalyst 1B contains 1 wt. % Pd,0.6 wt. % Au, 58 wt. % Ti and less than 20 ppm Cl.

EXAMPLE 2 Epoxidation Reactions

A 300 cc stainless steel reactor is charged with the supported noblemetal catalyst (0.07 g of 1A, 1B, or 1C), TS-1 powder (0.63 g), methanol(˜100 g), and a buffer solution (13 g of 0.1 M aqueous ammoniumphosphate, pH=6). The reactor is then charged to 300 psig with a feedconsisting of 2% hydrogen, 4% oxygen, 5% propylene, 0.5% methane and thebalance nitrogen (volume %) for runs utilizing a 2:1 O₂:H₂ ratio or afeed consisting of 4% hydrogen, 4% oxygen, 5% propylene, 0.5% methaneand the balance nitrogen (volume %) for runs utilizing a 1:1 O₂:H₂ratio. The pressure in the reactor is maintained at 300 psig via abackpressure regulator with the feed gases passed continuously throughthe reactor at 1600 cc/min (measured at 23° C. and one atmospherepressure). In order to maintain a constant solvent level in the reactorduring the run, the oxygen, nitrogen and propylene feeds are passedthrough a two-liter stainless steel vessel (saturator) preceding thereactor, containing 1.5 liters of methanol. The reactor is stirred at1500 rpm. The reaction mixture is heated to 60° C. and the gaseouseffluent is analyzed by an online GC every hour and the liquid analyzedby offline GC at the end of the 18 hour run. Propylene oxide andequivalents (“POE”), which include propylene oxide (“PO”), propyleneglycol (“PG”), and propylene glycol methyl ethers (PMs), are producedduring the reaction, in addition to propane formed by the hydrogenationof propylene.

The epoxidation results (see Table 1) show that mixed catalystcomprising TS-1 and either Pd—Bi/TiO₂ or Pd—Au—Bi/TiO₂ show asignificant increase in propylene selectivity resulting from reducedpropane make, as compared to mixtures of TS-1 and Pd-Au/TiO₂.

TABLE 1 Epoxidation Results O₂:H₂ Catalyst Propylene Catalyst RatioProductivity¹ Selectivity (%)² 1A 2 0.45 89 1C* 2 0.39 81 1A 1 0.41 861B 1 0.62 79 1C* 1 0.7 68 ¹Productivity = grams POE produced/gram ofcatalyst per hour. ²Propylene Selectivity = 100 − (moles propane/(molesPOE + moles propane)) * 100. *Comparative Example

1. A process for producing an epoxide comprising reacting an olefin,hydrogen and oxygen in the presence of a titanium or vanadium zeoliteand a supported catalyst comprising palladium, bismuth and a carrier. 2.The process of claim 1 wherein the titanium or vanadium zeolite is atitanium silicalite.
 3. The process of claim 1 wherein the titaniumzeolite is TS-1.
 4. The process of claim 1 wherein the supportedcatalyst contains 0.01 to 10 weight percent palladium and 0.001 to 5weight percent bismuth.
 5. The process of claim 1 wherein the carrier isselected from the group consisting of carbon, titania, zirconia, niobia,silica, alumina, silica-alumina, tantalum oxide, molybdenum oxide,tungsten oxide, titania-silica, zirconia-silica, niobia-silica, andmixtures thereof.
 6. The process of claim 1 wherein the olefin is aC₂-C₆ olefin.
 7. The process of claim 1 wherein the reaction isperformed in the presence of a solvent selected from the groupconsisting of alcohols, ethers, esters, ketones, nitrites, water, liquidCO₂, and mixtures thereof.
 8. The process of claim 7 wherein thereaction is performed in the presence of a buffer.
 9. A process forproducing propylene oxide comprising reacting propylene, hydrogen andoxygen in a solvent in the presence of a titanium zeolite and asupported catalyst comprising palladium, bismuth and a carrier, whereinthe supported catalyst contains 0.01 to 10 weight percent palladium and0.001 to 5 weight percent bismuth.
 10. The process of claim 9 whereinthe titanium zeolite is a titanium silicalite.
 11. The process of claim9 wherein the carrier is selected from the group consisting of carbon,titania, zirconia, niobia, silica, alumina, silica-alumina, tantalumoxide, molybdenum oxide, tungsten oxide, titania-silica,zirconia-silica, niobia-silica, and mixtures thereof.
 12. The process ofclaim 9 wherein the solvent is selected from the group consisting ofalcohols, ethers, esters, ketones, nitrites, water, liquid CO₂, andmixtures thereof.
 13. The process of claim 12 wherein the reaction isperformed in the presence of a buffer.
 14. A catalyst mixture comprisinga titanium or vanadium zeolite and a supported catalyst comprisingpalladium, bismuth, and a carrier.
 15. The catalyst mixture of claim 14wherein the titanium or vanadium zeolite is a titanium silicalite. 16.The catalyst mixture of claim 14 wherein the carrier is selected fromthe group consisting of carbon, titania, zirconia, niobia, silica,alumina, silica-alumina, tantalum oxide, molybdenum oxide, tungstenoxide, titania-silica, zirconia-silica, niobia-silica, and mixturesthereof.